Electrophoresis is the translation of charged objects in a fluid in response to an electric field. Electrophoretic inks are useful as a medium to enable bistable, low power types of displays. Electrophoretic displays have been developed using a dyed fluid and white particles sandwiched between parallel electrodes on top and bottom substrates. When an electric field is applied transverse to the substrates across the dyed fluid to translate the white particles to the viewing surface, the display appears white. When the electric field is reversed to translate the white particles away from the viewing surface, the display appears the color of the dyed fluid. Similarly, electrophoretic displays have also been developed using a clear fluid with two differently colored particles of opposite charge (e.g., positively charged white particles and negatively charged black particles) sandwiched between parallel electrodes on top and bottom substrates. When the electrode on the viewing side is charged negatively, the positively charged white particles are translated to the viewing surface, and the display appears white. When the electrode on the viewing side is charged positively, the negatively charged black particles are translated to the viewing surface, and the display appears black. The prior embodiments using parallel electrodes to translate particles transverse to the top and bottom substrates do not enable a transparent state. When the top surface is color A, then the bottom surface will appear color B, and vice versa.
A transparent state can be enabled by “in-plane” electrophoretic displays, in which electrodes are arranged to apply electric fields that are substantially parallel to the substrates to translate colorant particles through a clear fluid parallel to the substrates. This allows the colorant particles to be collected out of the viewing area of the display to create a transparent state. The colorant particles can also be spread across the viewing area of the display to create a colored state. Since the travel distances required for in-plane electrophoretic displays are typically much larger, the switching speeds are typically much slower. Reducing the travel distance is limited by the clear aperture of the viewing area for a given electrode width.
The application of electrophoretic inks in such displays has been hampered by limited mobility and uncontrolled hydrodynamic instability, both of which contribute to slow switching speeds. For in-plane electrophoretic displays, additional challenges include tightly compacting pigments when they are collected out of the viewing area and completely clearing them out of the viewing area to provide good contrast and brightness.